Electrical resistor and method of making same



Apnl 16, 1963 B. COHEN ETAL 3,086,186

ELECTRICAL RESISTOR AND METHOD OF MAKING SAME Fiied April 29, 1958 United States Patent O 3,086,186 ELECTRICAL RESISTOR AND METHOD F MAKING SAME Bernard Cohen, East Meadow9 and Benjamin G. Forman,

New York, N.Y., assignors to General Transistor Corporation, Jamaica, N.Y., a corporation of New York Filed Apr. 29, 1958, Ser. No. '731,809 Claims. (Cl. 338-237) This invention relates generally to the field of electrical resistance devices, and more particularly to an improved miniature precision resistor where resistance values are often very high.

With the appearance of certain metallic alloys including combinations of nickel, chromium aluminum and copper, it has become feasible to manufacture wires for resistance purposes of extremely fine diameter, of the order .0005 inch. Such Wire has an electrical resistance which remains fairly constant over a wide temperature range, of the order of 800 ohms per circular mil foot, so that using one-half mil diameter wire, the resistance obtained is 3200 ohms per foot. Such wire has a very low tensile strength, normally not over ten grams, so that breakage of the Wire and resultant shorting has been a substantial problem in the manufacture of resistors using this type of resistance wire.

It has been known in the art to form coils of such wire by Winding the same upon a coil form following which the coil form with the resistance wire is encased for protection. Since there are no materials from which the coil form may be made that are dimensionally stable, slight temperature changes will cause stresses upon the Wire, materially affecting the resistance values obtained, even when the dimensional change has not been sufficient to break the wire. It is also known in the art to embed coils of resistance wire of the type described in a synthetic resinous support, use being made of the so-called epoxy and/ or polyester resins in particular. While such construction offers a measure of protection against damage from external sources, the resin itself is sufliciently dimensionally unstable as to constantly subject the embedded wire to strains, so that a constant resistance value cannot be depended upon. The principal strain in such construction occurs where the epoxy resin, as a liquid is injected about the coiled wire in heated condition. As the resin solidifies, shrinkage varying from two to ten percent of the initial volume occurs, placing strains upon the coiled Wire which are maintained by the solidifying of the resin.

It has been found that coils of fine wire cannot be employed without some means for supporting the same in relatively fixed condition, since vibration, shock, and varying degrees of heat to which the device will be subjected when in use are often sufficient to either break the coil of wire or alter the resistance values obtained, which are in most cases relatively critical.

It is therefore among the principal objects of the present invention to provide an improved electrical resistor construction of the class described, in which the above mentioned difficulties have been substantially eliminated.

Another object of the invention lies in the provision of improved electrical resistor construction of the class described which may be manufactured at a reasonably low cost, and high reliability thereby permitting consequent wide sale, distribution and use.

A further object of the invention lies in the provision of a wire wound resistor of the class described in which all strains or stresses occurring in the wound coil have been relieved, and in which the same is encased in a heavy uid capable of supporting the coil, While permitting further strain relief.

Still another object of the invention lies in the provision of structure of the class described which may be formed using tools and techniques known in the art, and of materials which are readily obtainable.

A feature of the invention lies in the compact size of resistors of even the highest feasible values.

These objects and features, as well as other incidental ends and advantages, will become more clearly apparent during the course of the following disclosure, and be pointed out in the appended claims.

On the drawing, to which reference Will be made in the specification, similar reference characters have been employed to designate corresponding parts throughout the several views.

FIGURE 41 is a fragmentary view in elevation showing a first stage in the manufacture of a resistor formed according to the present invention.

FIGURE 2 is a fragmentary view in elevation showing a second stage in the manufacturing procedure.

FIGURE 3 is a side elevational view showing a third stage thereof.

`FIGURE 4 is a fragmentary elevational view showing a fourth stage thereof.

FIGURE 5 is a side elevational view showing a fifth stage thereof.

FIGURE 6 is a side elevational view showing a sixth stage thereof.

FIGURE 7 is an enlarged vertical central longitudinal sectional view showing a completed resistor.

FIGURE 8 is a vertical central longitudinal sectional view of the coil winding means shown on FIGURE l.

FIGURE 9 is a fragmentary sectional view as seen from the plane 9-9 on FIGURE 8.

In accordance with the invention, there is shown on FIGURE 8 a collapsible mandrel, generally indicated by reference character 10 mounted for rotation in a chucking means 11 in turn supported upon a conventional coil winding device (not shown). The chucking means includes a slidably mounted sleeve i12, a hollow shaft 13, supporting said sleeve, an outer collar 14 having pin means 15 engaging said sleeve, a coil engaging slotted plate 16 and resilient means 17 urging said collar 14 and plate 16 in a rightwardly direction as seen on FIGURE 8. Mandrel spreading means 18 is supported on the coil winding device by a bearing `19 and includes a recess forming member 20 as well as pointed means 21 engageable Within a slot Z2 on the mandrel 10. From a consideration of FIGURE 8, it will be apparent that when the recess forming member 20 and pointed means Z1 are moved to engage the mandrel 10, a substantial portion of the same will be expanded, as during a winding operation, While moving the same to the position shown on FIGURE 8 permits the relaxation of the mandrel so that a coil of Wire wound thereon may be readily removed.

The first step in the manufacturing procedure, is the positioning of the structure above described in the rel-a.- tion shown on FIGURE l, wherein the vertically disposed surface 23 of the recess forming member 20 cooperates with the exposed surface 24 of the plate 16 and the outer surface of the mandrel 10 generally indicated by reference character 25 to form a spool upon which the wire forming the resistance element of the resistor may be wound. A first end 26 of the wire 27 is obtained `from a source 28 such as a pivotally mounted spool, and is threaded through the slot 22 in the slotted plate yi16, care being taken to prevent the same from becoming entangled during subsequent Winding.

The wire coil generally indicated by reference character 30 is then Wound in a normal manner, counting means (not shown) preferably indicating the number of a turns made so that resistance value may be accurately determined.

Referring to FIGURE 2 in the drawing, the recess forming member 2t) is then moved rightwardly, lfollowing which a metallic protective cap 31, partially lled with silicone grease, is placed over the completed coil 3i), care being made to preserve the iirst end 26 and second end 32.

The can and coil are then removed from the mandrel by rotating the collar 14 to a point where the resilient means 17 gently slips the completed coil rightwardly as seen on FGURE 2, it being not necessary to touch the coil with the iingers.

The device at this stage has the general appearance shown on FIGURE 3 of the drawing, wherein one of the ends extends from the center of the coil 30, and the other of the leads extends along the outer surface thereof. The next step includes the injection by suitable means of a relatively heavy grease, as for example, a synthetic lubricant known in the art as silicone grease, to which has been added a small amount of powdered calcium carbonate to improve its electrical and thermal insulative properties. The grease is injected to completely fill the cap 31, so that the wound coil 3i? is suspended therein and insulated from the inner surface of the cap. Any strains imposed upon the coil during the winding operation thus have an opportunity to be relieved as movement of one turn or" the coil with respect to another is improved by the presence of the lubricant generally indicated by reference character 33.

Referring to FIGURE 5, the coil 30 is then maintained in position within the cap 31 by an insulating grommet 34, preferably formed of polyethylene Teon for synthetic resinous material having similar properties. In the iinal stage of assembly 'as shown on FIGURE 6, the ends 26 and 32 are electrically attached to leads 35 and 36, respectively which extend through a base member 37 in well known manner. 'IThe cap 31 is then welded to the inner surface 38` of the vbase member, whereby the iinished device lhas the appearance shown on FIGURE 7.

In completed condition, the resistor may then be subjected to the normal variations of temperature, shock and the like without substantial possibility of damage during use. As the coil 3G is suspended in relatively thick grease, any individual strains occurring with changes of temperature are readily accommodated, and the grease serves `as a cushion or shock absorber as well `as a means for dissipating heat. Thus the operating temperature of the coil is maintained at a substantially uniform level without further coolant.

It is to be understood that it is not considered that the invention lies within the precise details of structure shown and set forth in this specification, for obvious modications 4will occur to those skilled in the art to which the invention pertains.

What is claimed is:

l. An electrical resistor comprising: a planar base member, a hollow cap member integrated with said base member, a plurality of leads extending through said base member, a coil of resistance wire disposed Within said cap member, the ends of Iwhich are interconnected with said leads, and Ian electrically insulative viscous grease illing said cap member `and surrounding said coil, said coil being free of other support within said cap member. 2. An electrical resistor comprising: a planar base speelse member, a hollow cap member integrated with said base member, a plurality of leads extending through said base member, a coil of resistance wire disposed within said cap member, the ends of `which are interconnected with said leads, and an electrically insulative viscous silicone `grease lfilling said cap member and surrounding said coil, said coil being free of other support within said cap member.

3. The method of making an electrical resistor comprising the steps of: providing a planar metallic base member having electrical leads extending therethrough; providing a hollow cap member of `diameter substantially equal to that of said base member; winding a coil of small diameter resistance wire upon a mandrel to a desired resistance value and of a size to lit within said hollow cap member; placing said coil Within said cap member, and removing said mandrel; injecting sucient electrically insulative Huid to completely ll said cap member and support said coil; interconnecting the ends of said coil to said leads in said base member; sealing the open end of said cap member to said base member.

4. The method of making an electrical resistor comprising the steps of: providing a planar metallic base member having electrical leads extending therethrough; providing a hollow cap member of diameter substantially equal to that of said base member; winding a coil of small diameter resistance wire upon a mandrel to a desired resistlance value and of `a size to lit within said hollow cap member; placing said coil within said cap member, and removing said mandrel; injecting suflcient electrically insulative grease to completely ll said cap member and support said coil; interconnecting the ends of said coil to said lleads in said base member; sealing the open end of said cap member to said base member.

5. The method of making an electrical resistor comprising the steps of: providing a planar metallic base member having electrical leads extending therethrough; providing Ia hollow cap member of diameter substantially equal to that of said ibase member; winding a coil of small diameter resistance wire upon a mandrel to a desired resistance value and of a size to fit within said hollow cap member; placing 4said coil within said cap member and removing said mandrel; injecting suicient electrically insulative silicon .grease to completely iill said cap member and support said coil; interconnecting the ends of said coil to said leads in said base member; sealing the open end of said cap member to said base member.

References Cited in the le of this patent UNITED STATES PATENTS 1,684,204 Ruttenberg Sept. 11, 1928 2,119,292 Rollefson May 31, 1938 2,158,600 Wiegand May 16, 1939 2,287,460 Wagenhals et al. June 23, 1942 2,369,045 Hampton et al. Feb. 6, 1945 2,873,338 Elliott Feb. 10, 1959 FOREIGN PATENTS 1,046,431 France July 8, 1953v OTHER REFERENCES Silicones-a New Class of High Polymers of Interest to the Radio Industry, Proceedings of the I.R.E., July 1945, p. 445. 

1. AN ELECTRICAL RESISTOR COMPRISING: A PLANAR BASE MEMBER, A HOLLOW CAP MEMBER INTEGRATED WITH SAID BASE MEMBER, A PLURALITY OF LEADS EXTENDING THROUGH SAID BASE MEMBER, A COIL OF RESISTANCE WIRE DISPOSED WITHIN SAID CAP MEMBER, THE ENDS OF WHICH ARE INTERCONNECTED WITH SAID LEADS, AND AN ELECTRICALLY INSULATIVE VISCOUS GREASE FILLING SAID CAP MEMBER AND SURROUNDING SAID COIL, SAID COIL BEING FREE OF OTHER SUPPORT WITHIN SAID CAP MEMBER. 